Coatings and material layers are often applied to optical elements such as ophthalmic lenses, sunglasses, visors, windshields, windows, etc. for controlling the characteristics of light passing through these elements.
An exemplary light controlling device are “photochromic” sunglasses. This device's apparent color (the amount of light they absorb at a particular wavelength or range of wavelengths) reversibly changes in response to the intensity of light with which they are illuminated. Typically, the photochromic reaction is in response to bright ultraviolet illumination, while the enhanced absorption is at visible wavelengths. These devices rely on a reversible photo-induced chemical reaction in which a dye molecule absorbs ultraviolet photons, changes either chemically or conformationally, and the reaction product has an altered absorption characteristic of visible light. These familiar eyeglasses become dark in bright sunlight, and return to clear when indoors in a dimmer environment. These devices have the drawback that the degree to which the absorption changes is controlled entirely by the intensity of ambient ultraviolet light, and not by the wearer. Another drawback of these known devices is their perceived slowness in transitioning between dark and clear conditions.
Typical photochromic sunglasses take ten to fifteen minutes to revert from a dark state to a clear state. Notable prior art, U.S. Pat. No. 4,549,894, describes photochromic glass that regains a transmissivity 1.75 greater than it possesses in the fully darkened state 300 seconds after the activating illumination is removed. A variation on eyewear exhibiting this functionality exists, such as disclosed in U.S. Pat. No. 5,552,841, but it employs electro-optic means of controlling the light transmission in conjunction with electric-eye type devices.
In known photochromic devices, a photochromic dye and solvent mixture are prepared. This mixture is then applied onto an appropriate glass or plastic substrate such as an optical lens. This mixture imbibes or otherwise migrates into the substrate. Upon evaporation or removal of the solvent, the photochromic dye is retained by the substrate so that the lens switches between a transparent state and a colored state upon exposure to and removal of ultraviolet light. It is documented that such devices have a “transition-half time” of, at best, four to five minutes. Transition-half time is normally expressed as t1/2 and is defined as the time in seconds required for the device to return to an Optical Density of one-half the equilibrium value. Although such limited-feature devices have obtained acceptance in the market, it will be appreciated that the wide penetration into the market is hindered by this performance inadequacy.
Liquid crystal light shutters have also been developed as light transmission elements for eyewear. Some notable prior art is described in U.S. Pat. No. 4,279,474. In these devices, the electrically controllable birefringence of liquid crystals is exploited by sandwiching them between polarizers. In this implementation, the light transmissivity of the eyewear is controlled via an external electrical signal. Often, this signal is slaved to a photo sensor to produce responsive eyewear. A familiar example of such a device are the “automatic” windows in welding helmets that rapidly darken when a welding arc is struck, protecting the wearer's vision, as described in U.S. Pat. No. 4,039,254.
Attempts have been made to improve the aforementioned transition-half time by combining photochromic dyes with polymeric materials. However, these attempts have not improved the transition-half time and, in fact, are slower than the aforementioned imbibed device as disclosed in U.S. Pat. No. 6,773,108 B2. It is also known to use liquid crystal materials in combination with photochromic dyes as disclosed in U.S. Pat. No. 6,690,495. This disclosure reveals a marked improvement in the transition-half time. But, as with most all other liquid crystal devices, various other structural and processing features are required to enable such a device which can increase the cost and reduce the potential market size.
Other methods have been proposed to reduce processing, cost and transition half time. For example, devices using a combination of photochromic dyes with thermoplastic adhesives have been proposed. Cured adhesives, like polymers, can be categorized into thermoplastic or thermoset. Thermoplastic adhesives exhibit a glass transition temperature, Tg, above which the material can flow. This allows a material to be reversibly reshaped many times. Thermoplastics as a whole have many desirable properties suitable for optical applications including the ability to be injection molded and are considered the material of choice for this use. As such, thermoplastic adhesives have also been proposed as potential carriers of photochromic dyes. For example, Knox (U.S. Publication No. 2007/0177100 A1) and Gupta (International Publication No. WO 96/34735) both describe thermoplastic adhesives with impregnated photochromic dyes. Gupta further specifies a required Tg range for operation. These thermoplastic based devices demonstrate a marked improvement over existing products. Thermoset materials do not have a Tg, cannot be reversibly reshaped with an increase in temperature and cannot be injection molded. Furthermore, it was widely believed that use of a photochromic dye with a thermoset polymeric material would result in a significantly slower transition half time as compared with use of a photochromic dye in a liquid crystal material or thermoplastic materials. Accordingly, use of a thermoset adhesive in conjunction with a photochromic dye was not believed to provide any measurable benefit.
Therefore, there is a need in the art to provide a photochromic device that significantly improves the transition-half time over known photochromic devices. There is also a need for a photochromic device that does not require many of the components or structure normally associated with liquid crystal devices. And there is a need to manufacture such improved photochromic devices with readily available processing techniques which allow incorporation of the photochromic device with any number of optical elements.